The present invention relates to a compact electrical machine. In particular, it relates to the integration within the same volume of a magnetic gearing mechanism and an electrical generator/electrical motor.
FIG. 1 illustrates a wind turbine generator 2 having a housing 12 mounted on support 8 via swivel 10. The wind turbine generator 2 has a rotor 4 having a plurality of blades mounted on a low speed rotor shaft 16. A tail vane 6 attached to the housing 12 orients the wind turbine generator 2 in the wind so that the blades of the rotor 4 can drive the low speed rotor shaft 16 at low velocities, typically 60 rpm. The low speed rotor shaft 16 is supported using large bearings 14. The low speed rotation of the low speed rotor shaft 16 is converted into a high speed rotation of a generator shaft 20 using a mechanical gearbox 18. The generator shaft 20, which in this example is rotating at 1800 rpm, drives the electrical generator 22 which produces an electrical output 24. The use of a gearbox 18 to convert the low speed rotation of low speed rotor shaft 16 to the high speed rotation of the generator shaft 20 is particularly important in applications where an electrical generator 22 of reduced size is required, such as in wind turbine generators 2.
One problem with the mechanical gearbox 18, is that it requires maintenance. It has been suggested, in xe2x80x9cA novel high-performance magnet gearxe2x80x9d, K. Atallah et al, IEEE Transactions on Magnetics, vol. 37, no. 4, pt. 1, pp 2844-6, that if rare-earth magnets are used in a magnetic gearbox, then such a magnetic gearbox could be used as a substitute for the mechanical gearbox 18.
A cross-section of a magnetic gearbox is illustrated in FIG. 2. A high speed rotor shaft 20 of reduced diameter is supported within and coaxial with a cylindrical low speed rotor shaft 16 of larger diameter. The low speed rotor shaft 16 has a large number (P1) of permanent magnet pole-pairs 30 on the inner surface of the cylinder. The magnets 30 are attached to the inner surface of the steel low speed rotor shaft 16 and extend parallel to one another along a common axis of rotation 34 for the low speed rotor shaft 16 and the high speed rotor shaft 20. The magnets 30 are oriented so that the sources and sinks of flux are aligned perpendicularly to the axis 34. The orientation of the magnets 30 alternates. Every second magnet 30a has a source of flux directed towards the axis 34 of the low speed rotor shaft 16 and the adjacent magnets 30b have their sinks directed towards the axis 34 of the low speed rotor shaft 16. The high speed rotor shaft 20 has a smaller number (P2) of permanent magnetic pole-pairs 32 attached to its outer surface. The high speed rotor shaft 20 is coaxial with the low speed rotor shaft 16. The magnets 32 are oriented so that the sources and sinks of flux are aligned perpendicularly to the axis 34. The orientation of the magnets alternates. Every second magnet 32a has a source of flux directed towards the axis 34 of the high speed rotor shaft 20 and the adjacent magnets 32b have their sinks directed towards the axis 34 of the high speed rotor shaft 20. A large number (N) of stationary soft iron pole-pieces 36 are located between the exterior of the high speed rotor shaft 20 and the interior of the low speed rotor shaft 16 between the magnets 32 and 30. The stationary pole-pieces 36 are located at a fixed distance from the common axis 34 and are evenly distributed about that axis. The gearbox operates by locking one shaft""s magnetic field onto a space harmonic of the magnetic field created by the other shaft. The gear ratio is given in the simplest case by G=P2÷P1 when N=P1+P2. The low speed rotor shaft 16 when driven at a low speed causes the high speed rotor shaft 20 to rotate at a high speed, thus torque is transmitted from one shaft to the other at a fixed gear ratio.
It would be desirable to further improve electric machines which drive or are driven by gears.
According to one aspect of the present invention there is provided, an electrical generator arrangement comprising: a first plurality of permanent magnets collectively producing a spatially variable first magnetic field; a second plurality of permanent magnets arranged to rotate about a first axis; interference means positioned between the first plurality of permanent magnets and the second plurality of permanent magnets to interfere with the first magnetic field; motive means for moving the first plurality of permanent magnets and interference means relative to one another, to produce a second magnetic field for rotating the second plurality of permanent magnets; and a stator having windings arranged to transduce a changing magnetic field produced by the rotation of the second plurality of permanent magnets into electrical energy. According to one embodiment, the magnetic interference means are stationary and the first plurality of permanent magnets are mounted to rotate about the first axis.
According to this aspect of the present invention there is also provided a method of generating electricity comprising the steps of: interfering with a first magnetic field produced by a first plurality of permanent magnets to form beats (regular variations) in the first magnet field; using said beating first magnetic field to rotate a second plurality of permanent magnets; and transducing the time variable magnetic field produced by the rotating second plurality of permanent magnets into electrical energy.
According to another aspect of the present invention there is provided an electrical motor arrangement comprising: a first plurality of permanent magnets collectively producing a spatially variable first magnetic field; a second plurality of permanent magnets arranged to rotate about a first axis; interference means positioned between the first plurality of permanent magnets and the second plurality of permanent magnets to interfere with the first magnetic field and motive means for moving the first plurality of permanent magnets and interference means relative to one another, to produce a second magnetic field for rotating the second plurality of permanent magnets, said motive means comprising a stator having windings for transducing a changing electric field in the windings into the rotation of the first plurality of permanent magnets.
According to this aspect of the present invention there is also provided a method of transducing time varying electrical energy into the rotation of a motor shaft, comprising the steps of: using the time varying electrical energy to rotate a first plurality of permanent magnets; interfering with a first magnetic field produced by the rotating first plurality of permanent magnets to form beats in the first magnet field; and using said beating first magnetic field to rotate a second plurality of permanent magnets attached to the motor shaft.
The first magnetic field preferably varies regularly in space. In the air gap adjacent the second plurality of permanent magnets (without the interference means being present) it preferably has an approximately sinusoidal relationship to the radial angle with a period 2xcfx80/P1 where the first plurality of permanent magnets comprises P1 pole-pairs.
The interference means may regularly alternate between soft-ferromagnetic elements and non-ferromagnetic elements. The soft-ferromagnetic elements may be iron pole pieces, preferably of the same size with regular gaps between them such that they are evenly distributed at a fixed radius about the first axis. The first plurality P1 of permanent magnets and the second plurality of permanent magnets P2 preferably have a plurality N of pole-pieces located between them such that N=P1+P2. The relative motion between the first plurality of permanent magnets and the interference means produces beating in the first magnetic field.
Preferably, the first plurality of permanent magnets are attached to a soft-ferromagnetic support and the second plurality of permanent magnets are supported without attachment to soft-ferromagnetic material. The second plurality of permanent magnets may be bonded to a non-ferromagnetic support such as glass reinforced plastics (GRP).
The arrangement may have a radial flux geometry with the second plurality of permanent magnets arranged in a cylinder with an air gap of typically 3-5 mm between the stator and the cylinder. The first plurality of permanent magnets may be supported, using a first support, at a first fixed radius from the first axis and the second plurality of permanent magnets may be supported, using a second support and the first support, at a second fixed radius from the first axis, wherein a shaft and a plurality of bearings couple the first and second supports. Preferably, the first support and the second support rotate independently about the shaft on the plurality of bearings and the first support rotates relative to a housing on large bearings. The stationary pole pieces may be supported using a third supporting structure at a third fixed radius intermediate of the first and second fixed radii.
The support for the second plurality of permanent magnets may provide for air circulation over the stator when in use.
A preferred use of the arrangement is in a wind turbine generator having large bearings supporting the turbine rotor shaft, which is integral or attached to the support for the first plurality of permanent magnets.
A particular advantage of embodiments of the invention is that lubricating oil is not essential to the operation of the electrical machine. This is particularly attractive from an operating cost perspective for machines located at remote sites. Accordingly embodiments of the invention require little maintenance, are of compact size and of reduced weight.